Bacula is a set of computer programs that permit managing backup, recovery, and verification of computer data across a network of computers of different kinds. Based on Source Forge downloads, Bacula is the most popular Open Source backup program.

QuickFIX is the worlds first Open Source C++ FIX (Financial Information eXchange) engine, helping financial institutions easily integrate with each other.
The SVN repository is now locked. Latest code is hosted at github.
https://github.com/quickfix/quickfix

ChessDB is a free open-source chess database for Windows, Linux, Mac OSX and most UNIX and UNIX-like operating systems. ChessDB is based on code from Scid http://scid.sourceforge.net/ - a project for which there have been no updates for several years. In

WideStudio is an Integrated Development Environment (IDE) for building window applications in C++, Ruby, Python an Perl for Windows, MacOSX, Linux, FreeBSD, SOLARIS and other unix and BTRON and T-Engine. This is open and free (under MIT/X Consortiun Lice

XmlRpc++ is an implementation of the XmlRpc protocol written in C++, based upon Shilad Sen's excellent py-xmlrpc library. It is designed to make it easy to incorporate XmlRpc client+server support into C++ applications and requires no other libraries.

Alliance CAD System is a free set of EDA tools and portable cell libraries for VLSI design. It covers the design flow from VHDL up to layout. It includes VHDL simulator, RTL synthesis, place and route, netlist extractor, DRC, layout editor.

Xconq is an engine for turn-based strategy games. Variety of game genres run under Xconq: historical, sci-fi, fantasy, and modern. AIs. Network games. Multiple UIs and platforms. Easy-to-learn game design language. Large games library available

PTypes is a simple alternative to the STL that includes multithreading and networking. It defines dynamic strings, character sets, lists and other basic data types along with threads, synchronization primitives, named pipes and IP sockets.

ACE RADIUS library is a portable open-source implementation of RADIUS protocol, as defined in RFCs 2865 and 2866. It's written in C++ and is supported on a wide variety of POSIX-compliant platforms, including Linux and Solaris, and on Windows.

yaSSL, or yet another SSL, is an embedded ssl library for programmers building security functionality into their applications and devices. yaSSL is highly portable, and runs on standard as well as embedded platforms(QNX, ThreadX, VxWorks, Tron)